Nonflammable methyl nonafluorobutyl ether for electrolyte used in lithium secondary batteries

Use of nonflammable methyl nonafluorobuyl ether (MFE) has been studied to develop an inherently safe electrolyte for lithium secondary batteries. A no flash point (NFP) solution was prepared by mixing a proper amount of MFE with a common electrolyte solvent such as dimethyl carbonate or ethyl methyl carbonate (EMC) and the NFP electrolyte was obtained by dissolving organic lithium salts (e.g., Li[SO2C2F5](2); LiBETI) in the NFP solution. Cell capacities with NFP electrolyte of 1 mol dm(-3) (M) LiBETI-MFE/EMC (80:20 vol %) were limited by the charge-discharge process on the graphite anode. Electrolyte components were investigated in terms of modifying the solid electrolyte interface film to improve the charge-discharge performance and cycle life of NFP electrolyte. Adding cyclic carbonate (e.g., ethylene carbonate, EC) and LiPF6 to the electrolyte reduced the interfacial resistance in a graphite/Li cell. A 18650 cylindrical cell with EC and LiPF6 added to 1 M LiBETI-MFE/EMC (80:20) electrolyte discharged more than 90% of its capacity at a 1 C current rate (vs. the capacity at the 0.1 C) and kept more than 80% of its initial capacity after 560 cycles at the 1 C current rate and room temperature. Effects of these additives on charge-discharge capacities in a graphite/Li cell were also investigated in terms of electrochemical spectroscopy, X-ray photoelectron spectroscopy, solid-state Li-7 nuclear magnetic resonance, and attenuated total reflection infrared spectroscopy. (C) 2003 The Electrochemical Society.